Compartmentalised immersion heater comprising electrically insulating ceramic powder

Abstract

The invention relates to an immersion heater (100) configured to be brought into contact with a material to be heated (90), in particular a molten non-ferrous metal, the immersion heater comprising: - a sheath (120) defining a first compartment (150) that is at least partially filled with electrically insulating powder (130), the sheath being configured to be brought into contact with the material to be heated; - a wall (140) arranged inside the first compartment (150) and defining at least one second compartment (145), this second compartment being at least partially filled with an inert material; - a plurality of heating elements (115, 116) arranged in the electrically insulating ceramic powder (130) between the sheath (120) and the wall (140).

Description

DESCRIPTION Compartmentalized imprint heater with electrically insulating ceramic powder TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of foundry equipment and relates mainly to the foundry of non-ferrous metals having a relatively low melting point, typically below 1100°C.

[0002] The invention relates to an electric immersion heater for maintaining a previously melted metal bath in a liquid state or for melting a batch of solid metal, in particular alloys or non-alloyed metals of aluminum, magnesium or zinc. STATE OF THE ART

[0003] In the context of foundry equipment, an electric immersion heater is an electrical device which comprises a heating zone intended to be placed in contact with a bath of liquid metal to be heated, and a non-heating zone providing a connection between the heating zone and a power supply box.

[0004] In a foundry workshop, the most common use of an immersion heater is to maintain the temperature of a molten metal, previously melted using a gas furnace for example. For example, this temperature maintenance can be done in a liquid metal treatment ladle, during degassing or filtration of the liquid metal. According to a more recent application, designed by the applicant company, an immersion heater can be used to melt metal in the solid state and possibly to maintain the temperature of the molten metal thus obtained.

[0005] Immersion heaters for heating a bath of liquid metal have been known for decades, for example from GB 1 027 163, FR 2 720 888, they generally comprise one or more heating elements placed in a cylindrical sheath which separates them from the bath which they are to heat. As taught by FR 2 699 038 the sheath can be made of inert ceramic material, for example silicon nitride, boron nitride, SiAlON, silicon carbide. The heating element can be made in the form of a heating ceramic, for example SiC, as is known from WO 2005 / 060314, or graphite, as described in FR 2 559 886 and FR 2 622 382.

[0006] Immersion heaters for heating a bath of liquid metal are also known, the tubular sheath of which comprises a plurality of metal heating elements which are embedded in an electrofused magnesia powder, which is an electrical insulator and has suitable thermal conductivity. The immersion heaters currently on the market use this principle.

[0007] In a context of electrification of industrial methods and more particularly of foundry methods, notably with a view to limiting the use of more traditional heating methods based on the use of fossil fuels, there is a need for electrical devices providing electrical power, and therefore a heating capacity, greater than that offered by current devices.

[0008] To meet this need, several currently available immersion heaters can be used together in the same bath. Depending on the heating capacity required for a furnace, as many immersion heaters as necessary are used. Since the life of the sheaths in a liquid metal bath is limited, these immersion heaters must be replaced regularly. Increasing their number results in an increase in both investment and maintenance costs. The alternative to increasing the number of immersion heaters would be to provide immersion heaters with a larger capacity and size. However, increasing the size of immersion heaters comes up against certain technical and economic difficulties. OBJECTS OF THE INVENTION

[0009] The applicant company found that it was not possible to produce a high-power immersion heater at a reasonable price. Indeed, this increase in power is expensive due to the materials used in the structure of an immersion heater. As indicated above, an immersion heater usually comprises an inert tubular sheath which surrounds a core comprising electrically insulating ceramic powder, for example electrofused magnesia, in which the heating elements of the immersion heater and the electrical supply lines necessary for the heating are embedded. operation of these heating elements. Electrically insulating ceramic powder has a very high production cost. Therefore, increasing the volume of the immersion heater in order to position more heating elements or heating elements of larger size and power in said immersion heater induces a cost, associated with the necessary electrically insulating ceramic powder, which is too high. We are therefore looking for an alternative way to reduce the cost of immersion heaters, without reducing their quality and longevity.

[0010] Thus, the present invention aims to remedy the limitations described above and in particular to provide an immersion heater offering increased heating capacity, while limiting the cost associated with this increase in power.

[0011] To this end, according to a first object, the invention relates to an immersion heater configured to be placed in contact with a material to be heated, in particular a molten non-ferrous metal, which comprises: - a sheath delimiting a first compartment at least partly filled with electrically insulating ceramic powder, said sheath being configured to be placed in contact with the material to be heated, - a wall arranged inside said first compartment and delimiting at least one second compartment, the latter being at least partly filled with an inert material, - a plurality of heating elements arranged in the electrically insulating ceramic powder, between said sheath and said wall.

[0012] Thanks to these arrangements, a part of the first compartment, delimited by the sheath, and which, in the immersion heaters known from the prior art, is usually filled with electrically insulating ceramic powder, instead comprises an inert material or a gas whose cost is much lower than the cost of the electrically insulating ceramic powder.

[0013] According to the invention, the second compartment is filled at least in part with an inert material. The inventors discovered that the implementation of an immersion heater comprising a second compartment that is solidly empty had the effect of causing a thermal draft phenomenon. This thermal draft phenomenon occurs because the temperature difference between the bottom of the immersion heater with its resistances between 800°C and 1100°C and the top of the immersion heater which causes an air flow. The hot air, whose density and pressure are lower than those of cold air, rises in the tube. The addition of an inert material, and in particular a refractory ceramic insulator, in the second compartment makes it possible to limit this thermal draft effect.

[0014] In embodiments, the inert material at least partially filling the second compartment delimited by the wall is a refractory ceramic insulating material. Said material may be of the flexible type, in a machined block or vacuum-molded.

[0015] In embodiments, the inert material at least partially filling the second compartment delimited by the wall comprises high temperature resistant fibers. For example, these high temperature resistant fibers are polycrystalline mullite fibers or silico-alumina fibers.

[0016] In embodiments, the second compartment delimited by the wall is partly filled with an inert material and remains partly empty of solid material. Advantageously, the second compartment delimited by the wall is filled at least partly with an inert gas which may be argon or another noble gas.

[0017] In embodiments, the second compartment is at least 30% filled by volume with an inert material, the remaining volume of the second compartment being left empty of solid material. In other embodiments, the second compartment is at least 40%, 60%, 80% or 90% filled with an inert material.

[0018] An electric immersion heater has a heating zone and a non-heating zone that connects the heating zone to a heater holder connected to a power supply. The heating zone of the immersion heater is located on the lower part of the heater and has a plurality of heating elements, usually electrical resistors, heated to a high temperature by the passage of an electric current.

[0019] Advantageously, the part of the second compartment corresponding in height to the heating zone of the immersion heater is filled with inert material and the part of the second compartment corresponding in height to the non-heating zone of the immersion heater is left empty of solid material.

[0020] These provisions make it possible to obtain the effect of preventing thermal draft phenomena, while limiting the quantity of inert material required for the manufacture of the immersion heater.

[0021] In embodiments, the immersion heater comprises at least one thermocouple type temperature sensor. Advantageously, the at least one thermocouple is positioned in the first compartment of the immersion heater and more preferably in the heating zone of the immersion heater.

[0022] Thus, at least one thermocouple is arranged in the electrically insulating ceramic powder. Advantageously, the temperature sensor is protected by a thermowell.

[0023] In embodiments, the electrically insulating ceramic powder is selected from zinc oxide powder, alumina powder, magnesium oxide [magnesia] powder or boron nitride powder. It advantageously has good thermal conductivity.

[0024] In embodiments, the wall disposed within the sheath and defining a compartment filled at least in part with an insulating material is formed at least in part from a ceramic material.

[0025] Thanks to these provisions, the wall can be formed from a material of comparatively low cost compared to the cost of electrically insulating ceramic powder, while having sufficient mechanical strength and thermal resistance characteristics.

[0026] In embodiments, the wall is formed from a high-performance alloy, preferably a nickel-based alloy exhibiting low expansion at 1100°C. For example, the wall is formed from an alloy selected from Inconel alloys or CMSX single-crystal alloys.

[0027] A high-performance alloy or superalloy is a metal alloy with excellent mechanical strength and good creep resistance at high temperatures, good surface stability, and good resistance to corrosion and oxidation.

[0028] In embodiments, the sheath has a circular section. Advantageously, the sheath has an outer diameter greater than or equal to 50 millimeters, preferably greater than 75 mm, more preferably greater than 95 mm, and even more preferably greater than 115 mm.

[0029] Thanks to these provisions, the immersion heater has sufficient dimensions to include more heating elements or larger heating elements.

[0030] In embodiments, the wall is a tube of circular section. Advantageously, the center of the circular section sheath and the center of the circular section wall are substantially identical, so that the first compartment, formed between the sheath and the wall, and intended to be filled at least in part with electrically insulating ceramic powder, has an annular section.

[0031] Thanks to these arrangements, the heating elements can be arranged in said first annular section compartment. The heating elements thus positioned are close to the sheath, thus allowing better diffusion of heat towards the sheath and indirectly towards the material to be heated.

[0032] In embodiments, the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis running through the center of the sheath and in which the immersion heater comprises at least 15 turns, preferably at least 20 turns, preferably at least 20 turns, preferably at least 25 turns, preferably at least 30 turns. For example, the immersion heater comprises 36 turns.

[0033] A coil is a plurality of heating resistors supplied by the same current leads and forming a substantially linear assembly arranged longitudinally in the immersion heater, preferably close to the sheath.

[0034] As detailed above, the immersion heater which is the subject of the invention is particularly distinguished from known immersion heaters in that the diameter of the first compartment delimited by the sheath can be increased in order to accommodate more heating elements or larger heating elements. Thus, advantageously, the immersion heater comprises 16, 24, 32 or 36 turns.

[0035] In embodiments, the heating elements are arranged in substantially straight turns disposed substantially parallel to an axis running through the center of the sheath and wherein said turns are positioned near the inner periphery of the sheath.

[0036] Thanks to these arrangements, the coils positioned in this way are close to the sheath, thus allowing better diffusion of heat towards the sheath and indirectly towards the material to be heated.

[0037] In embodiments, the heating elements comprise electrical resistors comprising molybdenum or an alloy selected from an iron-chromium-aluminum alloy [FeCrAl], a nickel alloy and a chromium alloy.

[0038] Apart from the heating elements and the electrical connection elements to the outside, the device according to the invention advantageously has a cylindrical shape and a coaxial and symmetrical structure with respect to the central axis. Other embodiments can also be envisaged in which the device has a different shape and / or structure, for example an orthogonal section or a section of variable diameter.

[0039] A second object of the invention is represented by a method of manufacturing an immersion heater according to any one of the embodiments of the invention, in which a sheath, a tubular wall, a plurality of heating elements and an electrically insulating powder are supplied, said tubular wall is placed inside said sheath, the heating elements are introduced into the first compartment between the sheath and the wall, then said first compartment is filled with electrically insulating powder in stages, at least certain stages, and preferably each filling stage, being followed by at least one packing stage. BRIEF DESCRIPTION OF THE FIGURES

[0040] Other advantages, aims and particular characteristics of the invention will emerge from the following non-limiting description of at least one particular embodiment of the immersion heater which is the subject of the present invention, with reference to the appended drawings, in which:

[0041] [Fig 1] represents, schematically and in axial section view, a first embodiment of an immersion heater according to the invention,

[0042] Fig. 2] represents, schematically and in cross-sectional view, the immersion heater which is the subject of the first embodiment,

[0043] The reference numbers mentioned in the figures relate to: 90 a liquid to heat 80, 81 an arrow representing a heat flow 100 an immersion heater 110 a means of supplying electric current 115, 116 heating resistors 120 a sheath 130 electrically insulating ceramic powder 140 the tubular wall 145 a second compartment 150 a first compartment DETAILED DESCRIPTION OF THE INVENTION

[0044] This description is given without limitation, each characteristic of an embodiment being able to be combined with any other characteristic of any other embodiment in an advantageous manner.

[0045] Figures 1 and 2 show a first particular embodiment of an immersion heater 100 according to the invention. Figure 1 shows an axial sectional view of the immersion heater. Figure 2 shows a cross-sectional view of the heating part of the immersion heater 100, along the section plane AA, visible in Figure 1.

[0046] The immersion heater 100 is an electric immersion heater comprising a heating zone 101, and a non-heating zone 102 making it possible to make the connection between the heating zone and a support of the immersion heater 100 connected to an electrical power supply means 110. The immersion heater 100 is intended to be positioned in a tank [not shown] containing metal to be heated. The heating zone 102 of the immersion heater 100 is intended to be placed in contact with the metal, whether it is a molten metal to be maintained at temperature in order to keep it in the liquid state or a solid metal to be heated.

[0047] The heating zone 101 of the immersion heater 100 comprises a plurality of heating elements, 115, 116, brought to a high temperature by the passage of an electric current; this is a resistive heater. As will be explained in greater detail below, each heating element comprises an electrical conductor, which is in contact with an electrically insulating material 130 capable of transmitting its heat to a sheath 120 [outer sheath] forming the outermost layer of the body of the immersion heater 100, which in turn transmits the heat to the metal or alloy to be melted or maintained in the liquid state.

[0048] The mechanism for transmitting heat from the immersion heater sheath to the metal in the tank is most efficiently by conduction, when the immersion heater is immersed in liquid metal or when its sheath touches solid metal over a significant fraction of its surface. Outside of these areas of direct contact, the heat transfer between the sheath and the metal contained in the tank is radiative and / or convective. The heat transfer from the heating element 115 to the sheath 120 and then to the material to be heated 90 is represented by the arrow 80, in Figures 1 and 2.

[0049] The immersion heater 100 is for example configured to maintain in the liquid state a metal 90 in liquid form comprising a non-ferrous metal or an alloy with base of a non-ferrous metal which has a melting temperature low enough to be compatible with the use of an electric immersion heater. In particular, the non-ferrous metal may be any metal or alloy having a melting temperature which does not exceed approximately 1100°C. The base metal may, for example, be chosen from aluminum, zinc, magnesium, copper, tin, lead, lithium, silver.

[0050] The sheath 120 of the immersion heater 100 is preferably formed from boron nitride or silicon nitride. These refractory materials have high heat resistance and are chemically inert, in addition to offering satisfactory mechanical strength. The sheath 120 delimits a first compartment 150 inside which the heating elements, 115, 116, of the immersion heater 100 are arranged.

[0051] Said first compartment 150 is at least partly filled with electrically insulating ceramic powder 130. The electrically insulating ceramic powder 130 is a material selected for its electrical insulating characteristics, allowing direct contact with the electrical conductors of the heating element, and for its heat conducting characteristics. In addition, the electrically insulating ceramic powder 130 must have excellent temperature resistance, it must not degrade over time, allowing an effective lifespan of the immersion heater typically ranging from one to several tens of months of continuous use. It advantageously has good thermal conductivity.

[0052] According to an exemplary embodiment, the electrically insulating ceramic powder 130 is an electrofused magnesia [magnesium oxide: MgO] powder. The electrofused magnesia powder is manufactured by melting magnesium oxide at a temperature exceeding 3,000°C. In other embodiments, the electrically insulating ceramic powder is selected from a zinc oxide [ZnO] powder, an alumina [Al2O3] powder, or a boron nitride powder.

[0053] Inside the first compartment 150 delimited by the sheath 120, the immersion heater 100 comprises at least one wall 140 delimiting a second closed compartment 145, distinct from the first compartment. Said second compartment has the function of reducing the quantity of electrically insulating ceramic powder 130 necessary for the manufacture of the immersion heater 100 and thus of reducing the manufacturing cost. The wall 140 is typically a tubular wall, its axial cross-sectional section is advantageously circular.

[0054] The wall 140 is formed from a material having sufficient mechanical strength and thermal resistance characteristics, without requiring good thermal conduction. Preferably, the wall 140 is formed from ceramic, for example alumina. According to other examples of implementation, the wall 140 is formed from mullite or jcordièrit [Mji]|[TH2]. According to other embodiments, the wall is formed from a high-performance alloy, preferably a nickel-based alloy having low expansion at 1100°C. For example, the wall is formed from an alloy chosen from Inconel alloys or CMSX monocrystalline alloys.

[0055] In one embodiment, the second compartment 145 delimited by the wall 140 remains partly empty of solid material. The part of the second compartment empty of solid material comprises gas which may be air. Preferably, the air is expelled in order to be replaced by an inert gas, unlikely to oxidize the wall, which may be for example argon or another noble gas.

[0056] The second compartment 145 is at least partially filled with an inert material in particulate form. The inert material contained in the second compartment 145 makes it possible to prevent a thermal draft phenomenon. The inert material can also exert counter-pressure against the wall 140.

[0057] For example, said inert material comprises high temperature resistant fibers, for example polycrystalline mullite fibers or aluminosilicate fibers. According to another variant, said inert material is an alumina powder.

[0058] The sheath 120 typically has a circular cross-section. Advantageously, the outer diameter of the sheath 120 is at least 55 millimeters [hereinafter abbreviated as “mm”], preferably at least 75 mm. According to an alternative embodiment, the diameter of the sheath 120 is greater than 95 mm or greater than 115 mm. Increasing the diameter of the sheath 120 makes it possible to place a greater number of heating elements 115, 116 inside the immersion heater 100, which makes it possible to increase its thermal power.

[0059] In addition, the wall 140 is a tube of circular section also, forming a second compartment 145 of round section shape. The first compartment 150 formed on the one hand by the sheath 120 and on the other hand by the wall 140 thus has an annular shape advantageous for the placement of the heating elements, 115, 116, as detailed in the following paragraph. Preferably, the tube forming the wall 140 is of constant diameter over the entire height of the immersion heater.

[0060] Such alumina tubes are common products that exist on the market for other uses; therefore, their price is significantly lower than that of the quantity of electrofused magnesia powder [for example] saved by the insertion of such a tube into the device according to the invention.

[0061] The immersion heater 100 shown in Figures 1 and 2 comprises sixteen heating elements, of which only the heating elements 115 and 116 are labeled in Figures 1 and 2. In other embodiments, the immersion heater 100 comprises at least 20 heating elements, preferably at least 30 heating elements. Preferably and as illustrated in Figures 1 and 2, the heating elements are positioned close to the inner periphery of the sheath 120 in order to allow better heat transfer to the latter. In other words, if we consider a section of the immersion heater 100 as illustrated in Figure 2, the heating elements, 115, 116, are arranged in a circle concentric with the circle formed by the sheath 120. Any other arrangement and any other number of heating elements can be adopted without departing from the scope of the present invention.

[0062] Preferably, the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis running through the center of the sheath 120 and said turns are positioned close to the inner periphery of the sheath 120.

[0063] Preferably, the heating elements 115, 116 comprise electrical resistors comprising molybdenum or an alloy chosen from an iron-chromium-aluminium alloy [FeCrAl], a nickel and chromium alloy.

[0064] According to a particular embodiment [not shown], the tubular wall 140 forms a compartment with a rectangular or polygonal cross-section, for example octagonal or hexagonal.

[0065] According to a particular embodiment [not shown], the second compartment 145 formed by the wall 140 does not have the same dimensions over the entire height of the immersion heater 100. For example, the second compartment 140 at a cross-section in the non-heating part of the immersion heater 100 has a smaller surface area than the surface area of ​​the second compartment 140 at a cross-section in the non-heating part of the immersion heater 100. According to a particular embodiment [not illustrated], the immersion heater comprises a first compartment at least partly filled with electrically insulating ceramic powder and a plurality of walls arranged inside the first compartment, each of said walls delimiting a compartment which can remain empty of solid material or which can be filled at least partly with an insulating material.

[0066] Here we describe a method of manufacturing the immersion heater according to the invention. The inventors realized that good thermal contact between the conductors electrical conductors and the sheath 120 depends on the density of the bed of electrically insulating ceramic powder with which these conductors are in contact. According to the invention, a sheath 120, a tubular wall 140, a plurality of heating elements 115, 116 and an electrically insulating powder 130 are supplied, said tubular wall 140 is placed inside said sheath 120, the electrical conductors are introduced into the first compartment 150 between the sheath 120 and the wall 140, then said first compartment 150 is filled with electrically insulating ceramic powder in stages, at least some filling steps (and preferably each step) being followed by at least one packing step.

[0067] In a first embodiment, during this tamping step, the powder is pressed using a tamping tool in the form of an annular or ring section, capable of being inserted from above into the annular space formed by the first compartment 150. Thus, said first compartment is gradually filled with a dense, tamped bed of electrically insulating ceramic powder, which ensures excellent thermal contact with the heating elements 115, 116.

[0068] In a first variant of this method, each step of introducing the electrically insulating ceramic powder, or at least one of these steps, and preferably at least the last, is followed by a step of compacting by vibrating the sheath 150 and / or the tube 140.

[0069] In another variant of this method, said compacting tool is produced as a vibrocompaction tool.

[0070] These two variants can be combined.

Claims

CLAIMS

1. Immersion heater [100] configured to be brought into contact with a material to be heated (90), in particular a molten non-ferrous metal, characterized in that it comprises: - a sheath (120) delimiting a first compartment (150) at least partly filled with electrically insulating ceramic powder (130), said sheath being configured to be brought into contact with the material to be heated, - a wall (140) arranged inside the first compartment (150) and delimiting at least one second compartment (145), the latter being filled at least in part with an inert material, - a plurality of heating elements (115, 116) arranged in said electrically insulating ceramic powder (130), between said sheath (120) and said wall (140).

2. Immersion heater (100) according to claim 1, wherein the wall (140) disposed inside the sheath (120) and delimiting a second compartment (145) filled at least in part with an insulating material is formed at least in part from ceramic material.

3. An immersion heater according to one of claims 1 or 2, wherein said electrically insulating ceramic powder (130) is selected from a zinc oxide powder, an alumina powder, an electrofused magnesia powder, or a boron nitride powder.

4. Immersion heater (100) according to one of claims 1 to 3, in which said inert material at least partially filling the second compartment (145) delimited by the wall (140) comprises fibers with high temperature resistance, for example polycrystalline mullite fibers or silico-alumina fibers.

5. Immersion heater (100) according to one of claims 1 to 4, in which the second compartment (145) delimited by the wall (140) is partly filled with an inert material and remains partly empty of solid material.

6. An immersion heater (100) according to claim 5, wherein the immersion heater comprises a heating zone (101), housing the heating elements, and a non-heating zone (102) and wherein the part of the second compartment (145) corresponding in height to the heating zone (101) of the immersion heater is filled with inert material and the part of the second compartment (145) corresponding in height to the non-heating zone (102) of the immersion heater is left empty of solid material.

7. Immersion heater (100) according to one of claims 1 to 6, in which the sheath (120) has a circular section.

8. Immersion heater (100) according to claim 7, wherein the sheath (120) has an outside diameter greater than or equal to 50 millimeters, preferably greater than 75 mm, more preferably greater than 95 mm, and even more preferably greater than 115 mm.

9. Immersion heater (100) according to one of claims 1 to 8, in which the wall (140) is a tube of circular section.

10. Immersion heater (100) according to one of claims 7 or 8 and according to claim 9, in which the center of the sheath (120) of circular section and the center of the wall (140) of circular section are substantially identical, so that the first compartment formed between the sheath (120) and the wall (140) and intended to be filled at least in part with electrically insulating ceramic powder has an annular section.

11. Immersion heater (100) according to one of claims 1 to 10, in which the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis running through the center of the sheath (120) and in which the immersion heater comprises at least 15 turns, preferably at least 20 turns, preferably at least 20 turns, preferably at least 25 turns, preferably at least 30 turns.

12. Immersion heater (100) according to one of claims 1 to 11, in which the heating elements are arranged in substantially straight turns arranged substantially parallel to an axis running through the center of the sheath (120) and in which said turns are positioned close to the inner periphery of the sheath (120).

13. A method of manufacturing an immersion heater according to any one of claims 1 to 12, wherein a sheath (120), a tubular wall (140), a plurality of heating elements (115, 116) and an electrically insulating ceramic powder (130) are provided, said tubular wall (140) is placed inside said sheath (120), the heating elements (115, 116) are introduced into the first compartment (150) between the sheath (120) and the wall (140), and then said first compartment (150) is filled with electrically insulating ceramic powder in stages, at least some stages, and preferably each stage, of filling being followed by at least one stage of compaction.

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